Most GPS modules operate through TTL level RS-232 formatted serial. The messages they send usually conform to a standard called NMEA, or National Marine Electronics Association. These messages are in a relatively easy to parse format for microcontrollers, but there are some often overlooked caveats, and the actual reception and identifying of the messages in a raw serial data stream can be tricky unless you know what you are doing.
To this end the NMEA library has been written. It takes raw serial data, parses it, identifies the incoming messages, and parses them into meaningful values.
It is capable of receiving data through any serial connection based on the Stream class, so any HardwareSerial object (Serial, Serial1, Serial2, etc), and any Stream based SoftwareSerial objects.
Values are parsed all the time the process() function is called and stored internally until
required, and a full set of getter functions is provided to access all the data.